The present invention relates to a system for causing ablation of a target material of living tissue while not causing damage below a predetermined depth. The novel system is particularly useful using a carbon dioxide laser.
When using a laser for ablating tissue it is desirable to deliver maximum power density to the tissue to be ablated while minimizing temperature rise in adjacent tissue, particularly in the tissue underlying the tissue to be ablated preventing necrosis in such underlying tissue. Such a temperature rise in underlying tissue may cause thermal damage or carbonization, which generally results in increased scarring and healing time. For this purpose, surgical lasers used for tissue ablation are usually operated with short pulses to deliver high energy in short periods of time. Various pulsing techniques have been developed for this purpose, in which the energy applied for ablation is varied by changing the pulse repetition rate, pulse duration, and/or pulse energy.
Generally, it is desirable to provide power density of at least 40 watts/mm.sup.2 in order to obtain ablation. This power density must be provided, however, for a short enough period of time so the ablation is without carbonisation, and to minimize thermal damage below a depth of 50 micrometers. At the same time, it is desirable to have a spot diameter on the tissue of at least 3 mm to allow for controllable ablation, since a smaller diameter is more likely to produce holes rather than uniform tissue removal. In the pulse technique for operating a laser, however, these desirable characteristics oppose each other.
In this regard it is generally desirable to expose the tissue to pulses of less than 1 msec to minimize the depth of thermal damage, and to provide at least 0.1 sec between pulses to allow the tissue to cool down, while at the same time to provide an average power of not less than 20-30 watts to reduce the surgery time. However, in the pulse technique for operating a laser, these desirable characteristics also oppose each other.
Various prior art techniques are known is which a target material is scanned with laser radiation to selectively cause necrosis of the target material. Such prior art uses lasers that are absorbed nonuniformally by the target material so as to cause the selective necrosis. One such prior art teaching is U.S. Pat. No. 4,733,660 which issued to Irving Itzkan on Mar. 29, 1988 and is entitled Laser System for Proving Target Specific Energy Deposition and Damage.
A second such prior art teaching is in an article entitled Hexascan: A New Robotized Scanning Laser Handpiece by D. H. McDaniel et al, which appeared in Volume 45 of CUTIS page 300 in May of 1990.